The present invention relates to amplifiers and more particularly to preamplifiers for signals generated in a playback head.
Magnetic tapes containing ECG and other biological data are played back at speeded-up rates in the process of waveform monitoring/analysis, including Holter scanning. Typically the tapes will be played back at 60.times. and 120.times.. In a human ECG waveform, the real-time frequency range of interest is generally between about 0.03 Hz and 130-140 Hz; however, the low end of that frequency range at 60.times. is about 1.8 Hz and the upper end of that range at 120.times. is about 16 hKz. Characteristically the signal voltages generated in a tape playback head over which the magnetic tape passes are quite small, the actual signal voltage being on the order of microvolts and, more importantly, being a function of the frequency at which the signal is changing as it passes the head. In other words, because of the relatively wide range (i.e., 1.8 Hz-16 kHz) of frequencies encountered by the playback head, the resulting signal voltage gain will differ greatly from the low end of the frequency range to the high end, with the lower signal voltage gain appearing at the lower end of the frequency range and the higher voltage gain accordingly at the higher end of the frequency range.
It is preferable that the gain of the developed signal subsequently applied to various analytical and/or monitoring circuitry have a relatively uniform magnitude across the frequency range. While the small signals derived from the playback head might be applied to an amplifier or series of amplifiers which provided the necessary gain particularly at the low end of the frequency range, a further complication is introduced by the effects of noise inherently present in any such amplifying circuitry. More specifically, there is a type of noise known as 1/F noise which increases in density as the signal frequency decreases, yet it will be remembered that at such lower frequencies the signal voltage developed in the playback head is usually the smallest. This, therefore, leads to a situation in which the signal-to-noise ratio, particularly in the low frequency end of the band width spectrum, is intolerable.
Heretofore the prior art systems, in order to give a flat response across the frequency range, have used a plurality of selectable integrating networks connected to the output of a preamplifier in turn connected to the output of the tape playback head. The amplifier necessarily has been comprised of several stabilized amplifying units cascaded to give the needed gain and band width, followed by the integrating network selected for a particular frequency range which attenuates the amplifier output as frequency increases. This arrangement, however, is expensive, does not fully compensate for the effects of 1/F noise present in the amplifying circuitry, may be slow recovering from transient overload, and does not compensate for the stray capacitance of the head and wiring.
Therefore it is an object of the invention to provide, for signals generated in a playback head, amplifying circuitry including the playback head and capable of providing a relatively flat response over a wide frequency range and being relatively unaffected by 1/F noise in the lower portion of the signal band width.